1. Field of the Invention
The present invention relates to the conversion of chemical energy to electrical energy. More particularly, the present invention relates to a conductive substrate providing enhanced contact with an electrode active material. The improvement in electrode active contact is provided by chemically machining selected portions of the opposed major surfaces of the conductive substrate. The thusly fabricated substrate is particularly useful as a current collector in an electrochemical cell.
2. Prior Art
Presently, there are many different techniques for fabricating current collectors including subjecting a conductive foil substrate to mechanical expansion and perforating a conductive foil. However, mechanically expanded metal screens and perforated foils often have burrs which are potential contributors to battery shorting. Other typical state-of-the-art current collector designs, such as conventional chemical machining (etching or milling), electrolytic etching, and conductive foils provided with a vapor deposited bonding layer, attempt to increase the contact surface area without altering the generally planar configuration of the substrate workpiece. Maintaining the substrate having a planar surface structure limits the degree to which the contact surface area can be increased. In that respect, foil screen designs limit the variety of active material that can be contacted thereto. For example, adherence of pressed powders or flexible sheets of electrode active material to foil screens is virtually impossible. Finally, conventional woven fabric collectors are limited to minimum thicknesses required to maintain structural integrity. This detracts from improvements in reducing the passive volume of the fabric.
The present invention is, therefore, directed to a conductive substrate that serves as a current collector having improved contact or intimacy with an electrode active material without the problems characteristic of the various types of prior art collectors. The present current collector is provided by chemically etching a conductive foil to achieve the intimacy attributes of pulled mechanically expanded metal screens without burrs, to provide enhanced surface roughness characteristics that facilitate contact of the current collector with the electrode active material, to allow for support flexibility in selection of electrode material type, and to optimize reduction in the passive current collector material volume.
Thus, it is an object of the present invention to provide a current collector design that achieves increased intimacy between the electrode active material and the collector screen, which in turn yields increased discharge efficiency. When an electrochemical cell containing electrodes built with the present current collector is used to power an implantable medical device such as a cardiac defibrillator, this results in higher pulse voltages, reduced charging times and provides for increased discharge capacity, thereby extending the medical device life.
Another object of the present invention is to provide a current collector design that provides improved rapid delivery of energy capacity and prevents cell premature end of life.
A further object of the present invention is to provide a current collector design that allows enhanced electrode flexibility, facilitating winding into cells having a jellyroll electrode assembly.
Another object of the present invention is to provide a current collector that decreases the volume of the cell""s passive parts, thereby increasing the active material content.
Still another object of the present invention is to provide a current collector design that yields thinner finished electrodes facilitating cell assembly, and to provide a current collector design that renders wider use in applications with pulse amplitude requirements, such as implantable medical devices.
Furthermore, another object of the present invention is to provide a current collector that reduces cell internal resistance. The reduction in internal resistance is due to an enhanced intimate contact between the current collector and the supported electrode active material which consequently enables the fabrication of jellyroll electrode assemblies. Such electrode assemblies require less current collector leads in comparison to prismatic electrodes assemblies. The reduction of current collector leads facilitates cell manufacturability by eliminating additional joining operations while enhancing cell reliability.
According to the present invention, these objects are realized by chemically etching selected portions of the opposed major surfaces of the current collector to provide the screen with a xe2x80x9cbasket weavexe2x80x9d configuration.
These and other objects of the present invention will become increasingly more apparent to those skilled in the art by reference to the following description and to the appended drawings.